It is known that the emission of exhaust gases from combustion engines of vehicles contributes significantly to atmospheric pollution. Exhaust gases from both gasoline and diesel-fueled engines contain various combustion by-products for example, hydrocarbons, oxides of carbon such as carbon monoxide, and oxides of nitrogen and sulfur which are known to be harmful to the environment, particularly when emitted as micro-particulates. These especially harmful particulate contaminants are at their peak when combustion engines are run without sufficient oxygen to fully combust their fuel.
It is also known that an increased amount of these harmful combustion by-products will be produced if the combustion engines are run at low power level and/or operate at high temperature, during which efficiency of the engines will decline and possibly, lead to incomplete combustion of the fuel. Various systems such as catalytic converters and air injection systems have been developed which attempt to reduce the production and/or emission of these harmful by-products and also to increase efficiency of the combustion engines. In some modern engines, humid air such as steam has been injected into the combustion engines to improve performance of the engines and to reduce emission of the harmful substances.